Condenser lens-coupled photoconductive antenna device for terahertz wave generation and detection and fabricating method thereof

ABSTRACT

Provided are a condenser lens-coupled photoconductive antenna device for terahertz wave generation and detection and a fabricating method thereof. A condenser lens-coupled photoconductive antenna device for terahertz wave generation and detection includes a condenser lens, a photoconductive thin film deposited on the condenser lens, and a metal electrode formed on the photoconductive thin film for a photoconductive antenna. In the antenna device, the condenser lens and the photoconductive thin film are coupled to each other.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C 119(a) to KoreanApplication No. 10-2010-0098262, filed on Oct. 8, 2010, and KoreanApplication No. 10-2011-0097510, filed on Sep. 27, 2011, in the KoreanIntellectual Property Office, which is incorporated herein by referencein its entirety set forth in full.

BACKGROUND

Exemplary embodiments of the present invention relate to a condenserlens-coupled photoconductive antenna device for terahertz wavegeneration and detection and a fabricating method thereof.

The background art has been disclosed in Korean Patent No. 0645800(published on Nov. 14, 2006).

Terahertz waves are electromagnetic waves corresponding to a frequencyband between 0.1 and 10 THz. The terahertz waves lie between radio andlight waves, and have wavelengths shorter than those of millimeter wavesand longer than those of infrared waves.

Accordingly, since the terahertz waves have particular characteristicsdifferent from other electromagnetic waves, studies on the terahertzwaves have been conducted in various science fields, and the terahertzwaves will be applied to various industrial fields in the near future.However, terahertz wave generation and detection techniques haverecently developed, and most of the techniques require high-costequipment and up-to-date electronic and photonic technology. Among thesetechniques, a technique using a photoconductive antenna device based onan LT-GaAs thin film has been successfully developed and commercialized.As low-cost, small and light products have recently been introduced, theexpectation of marketability is increased.

FIG. 1 illustrates sequential fabrication processes a conventionalphotoconductive antenna device. The fabrication processes of theconventional photoconductive antenna device will be described withreference to FIG. 1.

First, a semi-insulating GaAs substrate 101 of about 2 inches diameteris prepared for deposition, and an LT-GaAs thin film 102 is deposited onthe GaAs substrate 101 by a deposition apparatus such as molecular beamepitaxy (MBE) or the like. In this case, it is important to prevent acrystal defect by control processing temperature and time andheat-treating temperature and time. After the deposition of the thinfilm 102 is completed, metal electrodes 103 for a photoconductiveantenna pattern are formed on a surface of the deposited thin film 102.Aluminum, gold or the like is used as a material of the electrode 103,and an alloy containing two or more metals may be used as the materialof the electrode 103 as occasion demands.

If the formation of electrode patterns is completed, the 2 inchsubstrate 101 is cut into individual chip with antenna patter, and acondenser lens 104 is then adhered to a rear surface of each of thesubstrates. In this case, it is important to exactly align centers ofthe metal pattern and the condenser lens. It is also important to allowthe substrate and the lens to be adhered closely to each other so thatvoid is not formed between the substrate and the lens. This is becausewhen an extremely small amount of void exists between the substrate andthe lens, scattering of terahertz waves occurs, and then noise isgenerated. Therefore, the entire performance of a system is lowered, andthe quality of spectrum and image obtained is deteriorated.

However, when the photoconductive material is attached to the condenserlens as described above, it is very difficult to exactly align thecenters of the electrode pattern and the condenser lens and to allow thesubstrate and the silicon lens to be adhered closely to each other sothat a void is not formed between the substrate and the silicon lens. Ina case where the aforementioned operation is not smoothly performed, thescattering of terahertz waves occurs due to void, and therefore, noisemay be caused in a terahertz signal. Since the semi-insulating GaAssubstrate made of a semiconductor material has a low penetration ratewith respect to the terahertz waves as compared with high-resistancesilicon, the signal to noise ratio (SNR) of the terahertz signal isdegraded.

SUMMARY

An embodiment of the present invention relates to a condenserlens-coupled photoconductive antenna device for terahertz wavegeneration and detection and a fabricating method thereof, which cansolve problems on an LT-GaAs-based photoconductive antenna device whichwidely used in the conventional photoconductive antenna and partiallycommercialized.

In one embodiment, a condenser lens-coupled photoconductive antennadevice for terahertz wave generation and detection includes a condenserlens, a photoconductive thin film deposited on the condenser lens, and ametal electrode for a photoconductive antenna, formed on thephotoconductive thin film. In the antenna device, the condenser lens andthe photoconductive thin film are coupled to each other.

In another embodiment, a fabricating method of a condenser lens-coupledphotoconductive antenna device for terahertz wave generation anddetection includes forming a condenser lens, depositing aphotoconductive thin film on the condenser lens, and forming a metalelectrode for a photoconductive antenna on the photoconductive thinfilm. In the method, the condenser lens and the photoconductive thinfilm are coupled to each other.

The condenser lens may be formed in a super-hemispheric shape and madeof high-resistive silicon.

The photoconductive thin film may be made of polycrystalline GaAs.

The depositing of the photoconductive thin film on the condenser lensmay be performed in the state that the condenser lens is mounted in asample holder for accommodating the condenser lens.

The sample holder may include a mounting portion having an insertionportion in which the super-hemispherical condenser lens is mounted, anda cover portion having a through-hole into which the super-hemisphericalcondenser lens is inserted.

The insertion portion of the mounting portion may be a hemisphericalconcave portion, and the radius of the through-hole may be decreased asthe through-hole approaches from the bottom to the top thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages will be moreclearly understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates a sequential fabrication processes of a conventionalphotoconductive antenna device;

FIG. 2 is a conceptual view illustrating a configuration of aphotoconductive antenna device for terahertz wave generation accordingto an embodiment of the present invention;

FIG. 3 illustrates a sequential fabrication processes of theconventional photoconductive antenna device according to the embodimentof the present invention; and

FIG. 4 illustrates a structure of a designed sample holder usedaccording to the embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to accompanying drawings. However, the embodiments are forillustrative purposes only and are not intended to limit the scope ofthe invention.

FIG. 2 is a conceptual view illustrating a configuration of aphotoconductive antenna device for terahertz wave generation accordingto an embodiment of the present invention. FIG. 3 illustrates asequential fabrication processes of the conventional photoconductiveantenna device according to the embodiment of the present invention. Theembodiment of the present invention will be described with reference toFIGS. 2 and 3.

As illustrated in FIGS. 2 and 3, the condenser lens-coupledphotoconductive antenna device according to this embodiment includes acondenser lens 201, a photoconductive thin film 202 deposited on thecondenser lens 201, and metal electrodes 203 for a photoconductiveantenna formed on the photoconductive thin film 202. The condenser lens201 and the photoconductive thin film 202 are coupled to each other.

The condenser lens 201 is formed in a super-hemispherical shape and madeof high-resistive silicon. The photoconductive thin film 202 is made ofpolycrystalline GaAs.

The operation of this embodiment configured as described above will bedescribed in detail with reference to FIGS. 2 to 4.

Photoconductive antenna devices are still fabricated at a very high costdue to technical difficulty and material price. Many studies for solvingsuch a problem have been carried out. Among these studies, a researchusing a polycrystalline GaAs thin film has been conducted into patents,and various applications using the research can be developed.

-   Korean Patent Application: 2009-0118339-   U.S. patent application Ser. No. 12/787,841-   Japanese Patent Application: 2010-139599

Details for properties of the terahertz waves, a principle of thephotoconductive antenna and characteristics of the polycrystalline GaAsthin film can refer to the patent applications. As disclosed in thepatent applications, the polycrystalline thin film can be grownregardless of the kind of substrate, and hence there is no conditionthat a GaAs single-crystalline substrate is necessarily used to grow theconventional LT-GaAs thin film. Therefore, the polycrystalline thin filmcan be grown on silicon, quartz, sapphire and glass, and then itspossibility has already been verified. Thus, it is possible to deposit aphotoconductive thin film directly on high-resistive silicon used as amaterial for the condenser lens without a substrate. Particularly, thehigh-resistive silicon is a material having a very high penetration ratewith respect to the terahertz waves. The high-resistance can minimizeabsorption of the terahertz waves onto the conventional semi-insulatingGaAs substrate, thereby obtaining strong terahertz wave signals.

FIG. 2 illustrates a configuration of the photoconductive antenna devicefabricated according to this embodiment and a principle of terahertzwave generation.

Referring to FIG. 2, the photoconductive antenna device according tothis embodiment includes the metal electrodes 203 for thephotoconductive antenna, the photoconductive thin film 202 and thecondenser lens 201. A femtosecond laser pulse 210 having a pulseduration of 10 to 100 fs is necessary to generate a terahertz wave. Thecondenser lens 201 with a super-hemispheric shape, made fromhigh-resistive silicon that is a material having a high penetration ratewith respect to the terahertz wave and having a high refractive index,is generally used to condense the generated terahertz wave in a certaindirection. Here, the super-hemispheric shape means a shape identical tothat of the condenser lens 201 illustrated in FIGS. 2 and 3. Thesuper-hemispheric shape means a shape further extended upward toapproach a spherical shape from a hemispherical shape.

The principle of terahertz wave generation will be described withreference to FIG. 2. If the femtosecond laser pulse 210 is incidentbetween the electrodes 203 to which a DC bias voltage from 10 to 50V isapplied, electron-hole pairs are generated in the photoconductive thinfilm 202, and the generated electric charges are moved to both theelectrodes by the bias voltage, thereby generating photocurrent. Thephotocurrent is flowed by a microwave pulse for a short time, and anelectromagnetic field is formed by a change in photocurrent. When themoving time of photoelectric charges is short as a pico-second or so, aterahertz wave 220 is generated by the electromagnetic field. Thegeneration and detection of the terahertz wave 220 is made to the entirespace. Since the dielectric constant of the photoconductive thin film202 and the condenser lens 201 is much greater than that in a freespace, most of the terahertz waves are emitted in the direction of thethin film. Therefore, the silicon condenser lens 201 is used to condensethe terahertz waves in one direction.

An antenna device for terahertz wave detection according to thisembodiment has the same structure and material as the antenna device forterahertz wave generation. However, in the antenna device for terahertzwave detection, the shape of an antenna electrode for detection may bechanged to improve detection characteristics.

FIG. 3 illustrates a fabricating method of the photoconductive antennadevice according to this embodiment. Here, it is unnecessary to preparea semi-insulating GaAs substrate formed from the conventional process,and a photoconductive thin film 202 is deposited directly on a flatsurface of a silicon condenser lens 201. Since the photoconductive thinfilm 202 is made of polycrystalline GaAs, MBE system is not necessarilyfor deposition of thin films and various methods such as organic metalchemical vapor deposition (MOCVD) or sputtering may be applied to thedeposition system.

If the deposition of the photoconductive thin film 202 is completed,metal electrodes 203 for a photoconductive antenna are formed. Since themetal electrode 203 is directly formed on the photoconductive thin film202 deposited on the silicon condenser lens 201, the process isperformed while aligning centers of the metal electrodes 203 and thephotoconductive thin film 202, without attaching the metal electrode 203to the photoconductive thin film 202 through a separate alignmentprocess. Accordingly, the number of substrates used can be decreased ascompared with the conventional method. As processes are simplified, timeand cost can be saved. Further, the misalignment between the metalelectrodes 203 and the silicon condenser lens 201 can be reduced,thereby obtain improvable effects.

That is, according to this embodiment, the polycrystalline GaAs thinfilm is directly deposited on the silicon condenser lens, so that it ispossible to considerably simplify the whole fabricating processes andprevent an error generation, thereby saving time and lowering cost.Further, it is possible to improve the performance and reliability ofthe photoconductive antenna device by simplifying processes and solvingan alignment problem. This becomes a basis for mass production whenterahertz systems are commercialized in the long term.

FIG. 4 illustrates a separated sample holder 400 necessary forperforming direct deposition on a flat surface of the silicon condenserlens related the process of FIG. 3. The sample holder 400 includes amounting portion 401 having an insertion portion in which thesuper-hemispherical condenser lens 201 is mounted, and a cover portion402 having a through-hole into which the super-hemispherical condenserlens 201 is inserted. The insertion portion of the mounting portion 401is a hemispherical concave portion, and the radius of the through-holeis decreased as the through-hole approaches from the bottom to the topthereof.

Since a thin film is basically deposited on a wafer in a generalsemiconductor deposition system, the sample holder 400 according to thisembodiment is separately required to load a condenser lens having acertain volume into the semiconductor deposition system. Since thesilicon condenser lens 210 has a super-hemispherical shape, the sampleholder 400 is divided into two components in this embodiment. If thecondenser lens 201 is first mounted in the hemispherical mountingportion 401 and the top of the mounting portion 401 is covered with thecover portion 402, the condenser lens 201 is not come from the sampleholder 400 even though the entire sample holder 400 is turned over. Thisis because the radius of the through-hole is decreased as thethrough-hole approaches from the bottom to the top thereof. In somesemiconductor deposition systems, the upper and lower portions of asample may be turned over while being mounted in a sample holder, whichcan be prevented by the sample holder 400 according to this embodiment.The size of the sample holder 400 may be formed suitable for the size ofthe condenser lens 201 to be used, and the diameter of the sample holder400 is generally from 10 to 12 mm.

According to the present invention, a polycrystalline GaAs thin film isdirectly deposited on a silicon condenser lens, so that it is possibleto considerably simplify fabrication processes and prevent an errorgeneration, thereby saving time and lowering cost. Further, it ispossible to improve the performance and reliability of thephotoconductive antenna device by simplifying processes and solving analignment problem. This becomes a basis for mass production whenterahertz systems are commercialized in the long term.

The embodiments of the present invention have been disclosed above forillustrative purposes. Those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

1. A condenser lens-coupled photoconductive antenna device for terahertzwave generation and detection, the antenna device comprising: acondenser lens; a photoconductive thin film deposited on the condenserlens; and a metal electrode for a photoconductive antenna, formed on thephotoconductive thin film, wherein the condenser lens and thephotoconductive thin film are coupled to each other.
 2. The antennadevice of claim 1, wherein the condenser lens is formed in asuper-hemispherical shape and made from high-resistive silicon.
 3. Theantenna device of claim 1, wherein the photoconductive thin film is madeof polycrystalline GaAs.
 4. The antenna device of claim 1, wherein thecondenser lens is formed in a super-hemispherical shape and made fromhigh-resistive silicon, and the photoconductive thin film is made ofpolycrystalline GaAs.
 5. A fabricating method of a condenserlens-coupled photoconductive antenna device for terahertz wavegeneration and detection, the method comprising: forming a condenserlens; depositing a photoconductive thin film on the condenser lens; andforming a metal electrode for a photoconductive antenna on thephotoconductive thin film, wherein the condenser lens and thephotoconductive thin film are coupled to each other.
 6. The method ofclaim 5, wherein the condenser lens is formed in a super-hemisphericalshape and made from high-resistance silicon.
 7. The method of claim 5,wherein the photoconductive thin film is made of polycrystalline GaAs.8. The method of claim 5, wherein the condenser lens is formed in asuper-hemispherical shape and made from high-resistive silicon, and thephotoconductive thin film is made of polycrystalline GaAs.
 9. The methodof claim 6, wherein the depositing of the photoconductive thin film onthe condenser lens is performed in the state that the condenser lens ismounted in a sample holder for accommodating the condenser lens.
 10. Themethod of claim 9, wherein the sample holder comprises: a mountingportion having an insertion portion in which the super-hemisphericalcondenser lens is mounted; and a cover portion having a through-holeinto which the super-hemispherical condenser lens is inserted.
 11. Themethod of claim 10, wherein the insertion portion of the mountingportion is a hemispherical concave portion, and the radius of thethrough-hole is decreased as the through-hole approaches from the bottomto the top thereof.